(23/1. The following extract gives the germ of what developed into an
interesting discussion in the "Origin" (Edition I., page 147). Darwin
wrote, "I suspect also that some cases of compensation which have been
advanced and likewise some other facts, may be merged under a more general
principle: namely, that natural selection is continually trying to
economise in every part of the organism." He speaks of the general belief
of botanists in compensation, but does not quote any instances.)

[September 1846].

Have you ever thought of G. St. Hilaire's "loi de balancement" (23/2.
According to Darwin ("Variation of Animals and Plants," 2nd edition, II.,
page 335) the law of balancement was propounded by Goethe and Geoffroy
Saint-Hilaire (1772-1844) nearly at the same time, but he gives no
reference to the works of these authors. It appears, however, from his son
Isidore's "Vie, Travaux etc., d'Etienne Geoffroy Saint-Hilaire," Paris
1847, page 214, that the law was given in his "Philosophie Anatomique," of
which the first part was published in 1818. Darwin (ibid.) gives some
instances of the law holding good in plants.), as applied to plants? I am
well aware that some zoologists quite reject it, but it certainly appears
to me that it often holds good with animals. You are no doubt aware of the
kind of facts I refer to, such as great development of canines in the
carnivora apparently causing a diminution--a compensation or balancement--
in the small size of premolars, etc. I have incidentally noticed some
analogous remarks on plants, but have never seen it discussed by botanists.
Can you think of cases in any one species in genus, or genus in family,
with certain parts extra developed, and some adjoining parts reduced? In
varieties of the same species double flowers and large fruits seem
something of this--want of pollen and of seeds balancing with the increased
number of petals and development of fruit. I hope we shall see you here